Element / isotope identity
Atomic number, isotope identity and discovery evidence are experimentally/evaluatively grounded.
Electron configuration
Very-heavy-element configurations are theory/evaluation-led; ordinary elements use evaluated ground-state references.
Bulk material structure
The Hexagonal close-packed (HCP) at room temperature reference is represented by a simplified teaching cell.
Phase / density data
Unknown values stay unknown; reported values are tied to the elemental material reference.
The lens describes evidence status, not confidence theatre. “Unknown” is kept unknown, and teaching schematics are not presented as direct measurements.
Thallium (Tl)
Thallium is element 81. This guide connects its periodic-table identity to evidence-aware structure, isotopes, uses, discovery and the search questions learners actually ask.
Thallium atomic number, mass, electron configuration and key properties
Thallium: quick answers
How many protons, neutrons and electrons does thallium have?
Thallium’s atomic number is 81, so every thallium atom has 81 protons, and a neutral atom also has 81 electrons. Its most common natural isotope, thallium-205, has 124 neutrons (other isotopes have different neutron counts).
What is the symbol for thallium?
The chemical symbol for thallium is Tl.
Is thallium a solid, liquid or gas at room temperature?
Thallium is a solid at room temperature (about 25 °C).
What family (group) is thallium in?
Thallium is a post-transition metal, in group 13, period 6 of the periodic table.
How many valence electrons does thallium have?
Thallium has 3 valence electrons, the electrons in its outer shell, which matches its position in group 13.
What is the electron configuration of thallium?
The ground-state electron configuration of thallium is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹.
From atomic number to chemistry
Read these as a chain of causes, not as isolated facts. Each step links to the concept hub if you want the underlying idea explained.
81 protons define thallium.
Ground-state/reference configuration frames atomic behavior; heavy-element predictions remain evidence-labelled.
Periodic position organizes trends without replacing element-specific evidence.
The teaching nucleus is one isotope, not the relative atomic mass.
Measured structures are visualized; unknown bulk structures stay unknown.
Thallium in its period and family
Thallium is element 81 in Period 6. Its p-block/series position provides useful periodic context, but element-specific experimental evidence takes priority over simple trend extrapolation.
Thallium Visual Lab
Explore Tl across the teaching nucleus, isolated-atom orbitals, evidence-aware material structure and temperature/evidence views, then connect those models to uses, isotopes, search-led questions and source-backed context.
Every mark points to one exact feature
The numbered markers explain the same information system used throughout Element Lookup. Unknown or predicted fields remain visibly labelled rather than being replaced with guesses.
Thallium in one minute
Atomic number 81 means every thallium nucleus has 81 protons.
The ground-state/reference electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹.
The representative teaching isotope is ²⁰⁵Tl.
Thallium is a metal with important +1 and +3 chemistry, but its toxicity dominates safe interpretation. Elemental Tl and thallium compounds must not be treated as interchangeable.
Material structure status: Hexagonal close-packed (HCP) at room temperature.
Shell rings organize electron counts. They are not electron trajectories or orbital shapes.
2 · 8 · 18 · 32 · 18 · 3 electrons
Displayed orbitals are isolated-atom, nonrelativistic teaching probability models. They are not bulk-band structures, bonding orbitals or direct measured electron-density maps; relativistic effects become especially important for very heavy elements.
The viewer represents the reviewed Hexagonal close-packed (HCP) at room temperature material reference for elemental Thallium. It is a teaching model, not a refined crystallographic coordinate set or a model of every compound.. The viewer is evidence-aware: measured structures are identified as such; unknown bulk structures stay unknown.
Teaching visualization; not a literal finite sample or thermal trajectory.What this model does—and does not—show
Displayed orbitals are isolated-atom, nonrelativistic teaching probability models. They are not bulk-band structures, bonding orbitals or direct measured electron-density maps; relativistic effects become especially important for very heavy elements.
Where do I meet thallium?
Clickable learning cards connect the element to materials, environment, technology, biology or research - depending on what the evidence actually supports.
Specialized optical and electronic materials
This context uses Thallium or a thallium-containing material; the element and its compounds/isotopes are kept distinct.
Thallium metal → engineered materials → evidence boundary
The same element can appear in very different materials; the page keeps elemental structure separate from compounds, alloys and isotope-specific applications.
Hexagonal close-packed (HCP) at room temperature
Elemental Thallium uses the reviewed ordinary structure shown in the Visual Lab.
Thallium in periodic context
Compare nearby or family-related elements without treating a periodic trend as a substitute for element-specific evidence.
| Atomic number | 80 |
|---|---|
| Context | neighbor / series |
| Atomic number | 81 |
|---|---|
| Context | neighbor / series |
| Atomic number | 82 |
|---|---|
| Context | neighbor / series |
Thallium properties: atomic, physical, thermal and chemical
Categories follow the science of this element rather than a fixed decorative template. Each row carries condition/provenance context and an evidence label; unknown values stay unknown.
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| Atomic number | 81 | Source-reviewed; see Sources below | Evaluated |
| Relative atomic mass | 204.38 | Source-reviewed; see Sources below | Evaluated |
| Ground-state electron configuration | [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹ | Source-reviewed; see Sources below | Evaluated |
| Group / period / block | Group 13 · Period 6 · p-block | Periodic-table placement | Evaluated |
| Electronegativity | 1.62 | Source-reviewed; see Sources below | Evaluated |
| Reference isotope | ²⁰⁵Tl | Source-reviewed; see Sources below | Evaluated |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| State context | Soft gray post-transition metal; highly toxic | Source-reviewed; see Sources below | Evaluated |
| Density | 11.8 g/cm³ | Source-reviewed; see Sources below | Evaluated |
| Material / molecular structure | Hexagonal close-packed (HCP) at room temperature | The viewer represents the reviewed Hexagonal close-packed (HCP) at room temperature material reference for elemental Thallium. It is a teaching model, not a refined crystallographic coordinate set or a model of every compound. | Measured |
| Classification | Post-transition metal | Periodic-table / chemistry classification | Evaluated |
| Structure-model scope | The viewer represents the reviewed Hexagonal close-packed (HCP) at room temperature material reference for elemental Thallium. It is a teaching model, not a refined crystallographic coordinate set or a model of every compound. | Teaching visualization; exact crystallographic coordinates are not implied unless stated. | Reviewed |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| Melting / transition reference | 577 K | Source-reviewed; see Sources below | Evaluated |
| Boiling / gas reference | 1746 K | Source-reviewed; see Sources below | Evaluated |
| Phase-path context | At approximately standard pressure, thallium is treated as solid below 577 K, liquid between the melting and boiling references, and gas above 1746 K. Solid-state allotropy is only shown where explicitly reviewed. | Shared phase registry drives the slider, regions and markers. | Evaluated |
| Condition warning | Temperature and pressure define phase behavior; purity/allotropy may matter. | Teaching condition statement | Reviewed |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| Ordinary electrical behavior | Metallic conductor | Qualitative bulk behavior; exact resistivity depends on temperature, purity and alloy state. | Measured |
| Conduction model | Collective solid-state electrons | Do not interpret isolated-atom orbital clouds as literal current paths. | Reviewed |
| Surface / compound caveat | Oxides, salts and alloys can behave differently from the pure metal | Material context | Reviewed |
| Engineering values | Condition-dependent | Use condition-specific materials data for engineering calculations. | Reviewed |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| Common oxidation states | +1, +3 | Source-reviewed; see Sources below | Evaluated |
| Ion / common ion context | Tl³⁺ | Source-reviewed; see Sources below | Evaluated |
| Periodic chemistry context | Thallium is element 81 in Period 6. Its p-block/series position provides useful periodic context, but element-specific experimental evidence takes priority over simple trend extrapolation. | Element-specific interpretation | Reviewed |
| Chemistry caution | Elemental form, ions and compounds are distinct chemical objects. | Interpretive teaching rule | Reviewed |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| ²⁰⁵Tl | Reference teaching isotope | Mass number belongs to a specific isotope and is not the same thing as relative atomic mass. | Evaluated |
| Isotope evidence | Element-specific nuclear context | Half-life and decay properties are isotope-specific; the page does not generalize one isotope to all atoms. | Evaluated |
| Teaching nucleus | ²⁰⁵Tl · 81 protons + 124 neutrons | Reference isotope used in the nucleus model | Reviewed |
Is Thallium a solid, liquid or gas? State at temperature
At approximately standard pressure, thallium is treated as solid below 577 K, liquid between the melting and boiling references, and gas above 1746 K. Solid-state allotropy is only shown where explicitly reviewed.
Where on Earth is Thallium found or produced?
Who discovered Thallium, and when?
William Crookes discovered thallium spectroscopically in London; Claude-Auguste Lamy soon isolated metallic thallium.
The element name and discovery story are part of the historical record; search-led questions are answered without turning history into scientific evidence for bulk properties.
Current use is described at the level supported by the element’s availability and evidence, with research-only elements kept research-only.
From source material to Thallium applications: high-level material path
Thallium enters supply chains through ores, by-products or specialized refining routes rather than through the educational structure shown here.
Industrial separation/refining produces metal or element-specific compounds; this guide does not provide operational extraction recipes.
The refined material is converted into the particular alloy, compound, device or catalyst needed by the application.
Recycling and recovery depend on host material and economics; application materials must not be confused with pure element.
What is thallium used for?
Specialized optical and electronic materials
Element-specific use context; compounds/alloys are distinguished from pure metal.
High-refractive-index and low-melting glasses via thallium compounds
Element-specific use context; compounds/alloys are distinguished from pure metal.
Historical/specialized low-temperature alloy applications
Element-specific use context; compounds/alloys are distinguished from pure metal.
Thallium isotopes and natural abundance
²⁰⁵Tl
Reference teaching isotopeMass number belongs to a specific isotope and is not the same thing as relative atomic mass.
Isotope evidence
Element-specific nuclear contextHalf-life and decay properties are isotope-specific; the page does not generalize one isotope to all atoms.
Five-question Thallium check
What is Thallium’s atomic number?
Which statement best describes the material evidence for Thallium?
What is the safest rule for Thallium uses?
Thallium questions students commonly ask
Each answer starts with the direct fact, then explains the chemistry, evidence or material context so the result is understandable rather than merely memorized.
What is the atomic number of thallium?
Short answer: The atomic number is 81, meaning every thallium nucleus has 81 protons.
Atomic number is defined by proton count, so 81 protons are what make an atom thallium. A neutral thallium atom also has 81 electrons, while isotopes can have different neutron counts without changing the element.
Key point: Atomic number = proton count.
What is the symbol for thallium?
Short answer: The chemical symbol is Tl.
The symbol Tl is the standardized chemical abbreviation for element 81. In a chemical formula, Tl identifies thallium atoms; a compound containing Tl is not automatically the same material as elemental thallium.
Key point: Tl always identifies element 81.
What is thallium?
Short answer: Thallium is element 81, a soft post-transition metal in Group 13.
Atomic number 81 means every thallium nucleus contains 81 protons. In the periodic table, Thallium is classified here as a post-transition metal in Period 6 and Group 13. Thallium is element 81 in Period 6. Its p-block/series position provides useful periodic context, but element-specific experimental evidence takes priority over simple trend extrapolation.
Key point: Tl is element 81; its periodic position and electron structure explain the rest of the page.
Is thallium a metal?
Short answer: Yes. Thallium is a metal, but it and many thallium compounds are highly toxic.
This guide classifies Thallium as a post-transition metal. Its periodic position is Period 6, p-block, Group 13. Thallium is element 81 in Period 6. Its p-block/series position provides useful periodic context, but element-specific experimental evidence takes priority over simple trend extrapolation.
Key point: Periodic-table classification describes recurring atomic/chemical behavior; it does not make every element in a column physically identical.
What is thallium used for?
Short answer: Modern uses are specialized and often involve thallium compounds in electronics, optics or glass rather than bulk thallium metal.
Specialized optical and electronic materials: Element-specific use context; compounds/alloys are distinguished from pure metal. High-refractive-index and low-melting glasses via thallium compounds: Element-specific use context; compounds/alloys are distinguished from pure metal. Thallium is a metal with important +1 and +3 chemistry, but its toxicity dominates safe interpretation. Elemental Tl and thallium compounds must not be treated as interchangeable.
Key point: Always distinguish the pure element from the compound, alloy, isotope or device material that actually performs the application.
Scientific sources for Thallium
- Royal Society of Chemistry - Thallium
- NIST - Atomic Weights and Isotopic Compositions
- IUPAC - Periodic Table of Elements
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